Why two "exhaust stacks" for some steam generators?

It seems like the roofs of most steam generator equipped F units have two vents, or whatever. Clearly, one is the exhaust for the "fire". But I wonder what the other is. I had been assuming it was an air intake. But someone pointed out that the whole carbody was pretty open, so there was no need to bring in outside air from the roof. In addition, GN apparently did some of their own steam generator installations, and those have only the single exhaust.

So, I'm asking if anyone knows what's with the "extra" exhaust. Or intake. Or whatever.

I am not turning up anybody who "officially" knows, both here and elsewhere.

Photos and drawings of Vapor-Clarkson generators show an exhaust vent and an intake vent for a forced draft for the fire. If this was typical, then there were two stacks/pipes/whatever.

It appears that, typically, both the exhaust and the air inlet were piped up through the roof (hatch). But GN, typically, did not pipe the air inlet that way. It appears that they used a free air inlet inside the locomotive.

Why either choice was made is still a mystery. I don't see the gain in sucking air from outside, at the roof--only thing that comes to mind would be fan noise. And I do see downsides.

I am coming into this late. Are we talking about the equipment on an F-unit roof that would be represented in a model by this?

If so: the rectangular part is the stack, and the two-piece round part is the air intake. While the 'round thing' may look like the vent for a home gas furnace or water heater, the Vapor-Clarkson combustion always involves a considerable forced draft when running and I expect a domed sheet-metal 'exhaust' would lose its paint (or worse!) in fairly short order...

Some locomotives that were 'winterized' drew the combustion air from inside the carbody, the principal drawback being the required airflow to the generator would upset other air circulation in the unit's carbody (of course these were long before the 'pressurized-engine-room' days). It has been a long time since I saw a Vapor-Clarkson installed in something like this, but if I recall correctly there were provisions in the fairly detailed operating placards against flashbacks when starting the unit 'flooded' with oil. This would be no fun, even with wire flame arrestors, inside an F-unit engine room...

The ones I'm familiar with are round where they go through the roof as well; most of the pictures I recall seeing of generators running in service have what appear to be 'cylindrical' exhaust when you see the exhaust lifting. (Remember that these are once-through monotubes and do not require 'pops' the way most locomotives do, so those white plumes you see in some photos are likely combustion-exhaust water, not steam that would have to be vented outside. There was at one time a small relief jet into the stack that was intended to help stop short cycling, but I believe it was gone by 1948)

Note that in your picture the 'stack' should be through-drilled: it is a hole with a pipe through it, although I do not remember what arrangements are made to 'catch' environmental water going down it before it ponds in the OK-4625 or whatever exhaust plenum.

I don't have time to find a detail shot from 'above' of the stack on a GG1, but it should be very similar; it never occurred to me to check whether the boiler combustion air came from outside or inside!

Can anyone with a link to an online copy or scan of Vapor Heating Corporation manual 1-208 and/or 2-203 please post it here? I suspect this will help determine what is intended to do what.

It seems like the roofs of most steam generator equipped F units have two vents, or whatever. Clearly, one is the exhaust for the "fire". But I wonder what the other is. I had been assuming it was an air intake. But someone pointed out that the whole carbody was pretty open, so there was no need to bring in outside air from the roof. In addition, GN apparently did some of their own steam generator installations, and those have only the single exhaust.

So, I'm asking if anyone knows what's with the "extra" exhaust. Or intake. Or whatever.

Ed

I have a pdf of an EMD operators manual for an E8. It has diagrams identifying all the external fittings.

It clearly labels the rear most dome on the steam generator roof panel as the "boiler air intake" and the other as the "boiler stack".

A fair number of roads bought F3's with steam generators, only a few bought F7's with steam generators.

ATSF bought ABBA sets, steam generators in the B units, extra water in that space in the A units.

Steam generators came in a number of sizes, from at least two different suppliers, so exact position of vents/intakes could vary.

Burlington E units had two smaller steam generators, giving them two intakes and two exhaust vents on the roof. I don't know if any other E's were built this way.

IIRC, GN and NP had F unit sets with various set ups, some A units with, some without like the ATSF. The NP also had two FP7's.

The introduction of the FP7 corrected the water storage problem of steam equiped/dynamic brake equiped F series A units, a fair number of steam equiped F7B units were built to run in sets with the FP7's.

Taking the combustion air from inside the car body would likely present a number of problems, dispite the open nature of an F or E unit.

External air would have a more consistant temperature, and be cooler and dense, beter for combustion.

Note that in your picture the 'stack' should be through-drilled: it is a hole with a pipe through it, although I do not remember what arrangements are made to 'catch' environmental water going down it before it ponds in the OK-4625 or whatever exhaust plenum.

I don't have time to find a detail shot from 'above' of the stack on a GG1, but it should be very similar; it never occurred to me to check whether the boiler combustion air came from outside or inside!

.......

On an HO model, like the one pictured, drillng that hole thru would be impractical on several levels. It would simply expose the gear box and wiring to some limited view, and the hole would likely collect dust with time.

On an HO model, like the one pictured, drillng that hole thru would be impractical on several levels. It would simply expose the gear box and wiring to some limited view, and the hole would likely collect dust with time.

But it would have the advantage of looking prototypical; that shell with the simplistic molded surround doesn't. (Of course you don't leave the hole open, you put a section of appropriate thin-wall tubing in its center and weather it to resemble the exhaust duct... with the inside lower part blackened and closed shy of fouling any of the mechanism.)

This is one of those things like drilling out the mouths of horn bells that gives relatively high return for the effort if you care about close-up appearance. (Yes, I know, this is more appropriate for the MR forum, but the general idea can be discussed here...)

On an HO model, like the one pictured, drillng that hole thru would be impractical on several levels. It would simply expose the gear box and wiring to some limited view, and the hole would likely collect dust with time.

But it would have the advantage of looking prototypical; that shell with the simplistic molded surround doesn't. (Of course you don't leave the hole open, you put a section of appropriate thin-wall tubing in its center and weather it to resemble the exhaust duct... with the inside lower part blackened and closed shy of fouling any of the mechanism.)

This is one of those things like drilling out the mouths of horn bells that gives relatively high return for the effort if you care about close-up appearance. (Yes, I know, this is more appropriate for the MR forum, but the general idea can be discussed here...)

All true, but many of today's models actually don't have the room inside for that. Even Athearn's long time ago tooled F unit has a big cast metal weight you would hit within 1/4". Weights, speakers, gear towers, wiring are carefully designed to take advantage of every cubic mm inside the shell.

But a little flat black paint in the hole, and some "exhaust" weathering does look pretty good for all but the closest examination.